1. Field of the Invention
The present invention relates to a thin film forming apparatus and, more specifically, to a thin film forming apparatus which forms a thin film over the surface of a substrate by spraying an atomized source solution over the surface of the heated substrate.
2. Description of the Prior Art
A conventional thin film forming apparatus of a spraying system sprays a source solution prepared beforehand over the surface of a substrate to form a thin film over the surface of the substrate. The thin film forming apparatus is provided with an atomizer for atomizing the previously prepared source solution, a nozzle disposed above the atomizer to spout the atomized source solution into a film forming chamber, and an exhaust duct connected to the film forming chamber. The atomizer comprises a mist box and a sprayer disposed in the mist box. Only a mist of comparatively small droplets of the source solution is spouted through the nozzle into the film forming chamber, and then the mist is exhausted from the film forming chamber through the exhaust duct.
Substrates arranged successively in a line are moved at a given speed from a preheating chamber through the film forming chamber to an exit. While being moved through the film forming chamber, the substrates are heated with a heater placed opposite to the backsides of the substrates. The mist of the source solution touches the surfaces of the heated substrates as the same move through the film forming chamber, a material contained in the solution reacts with oxygen contained in air or in the moisture of the source solution to form thin films of an oxide over the surfaces of the substrates. The substrates having their lower surfaces coated with the thin films are delivered from a substrate delivery chamber.
Since the proportion of liquid ingredient and gaseous ingredient contained in the mist of the source solution is liable to change while the mist spouted through the nozzle into the film forming chamber flows through the film forming chamber the exhaust duct, mists of different states are formed which deposit films of different characteristics on the substrate.
That is, the mist of the source solution as spouted into the film forming chamber is a perfect mist, namely, a mist consisting of liquid droplets, and hence a film deposited from such a perfect mist consists of crystals oriented to the plane (200) on a three-dimensional coordinate system, has a comparatively smooth surface, and has a comparatively high electrical resistance because the film contains impurities, such as Cl and F, excessively.
On the other hand, most droplets of the mist are vaporized in the vicinity of the exhaust duct and hence a film deposited from such a mist consists of crystals oriented to various planes including the plane (200), has a comparatively coarse surface, and has a comparatively low electrical resistance because the film contains only a small amount of impurities.
Although thin films of different characteristics are formed respectively at different positions in the film forming chamber, a film having the former characteristics and a film having the latter characteristics are formed successively over the surface of the substrate to form a thin film because the substrate is moved in one direction.
Some purposes require a thin film of a construction other than such a layer construction, however, the conventional thin film forming apparatus is unable to form a thin film meeting such a requirement.
Most portions of a thin film formed over the surface of a substrate is formed at a very high film forming rate in the vicinity of the spouting opening of the nozzle, at which the mist is spouted toward the surface of the substrate, and the film forming rate decreases sharply with the distance from the spouting opening of the nozzle. Therefore, the moving speed of the substrate must be comparatively low to form a thin film having a comparatively large thickness. However, reduction in the moving speed of the substrate entails the reduction of the productivity of the thin film forming apparatus.